1907 



Jennings.— Behavior of the Starfish. 67 



why sometimes one of these movements takes place, sometimes 

 another, and the rules followed in the actions. We will take 

 up each act or set of acts separately. 



The Attack.— What causes the rosettes of pedicellariae sur- 

 rounding the spines to rise to the attack 'I This is usually caused, 

 as we have seen, by some creature which gets among the gills and 

 spines, and disturbs them. But there are many conditions on 

 which it depends. 



As a I'ule the pedicellarrae do not rise unless there are re- 

 peated stinndi. If an organism touches the starfish but once, or 

 if we thrust a bristle against the gills or against any of the struc- 

 tures on the upper surface of the starfish, there is either no 

 response to the first touch or merely a slight movement of the 

 pedicellariae. At the second the rosettes may show slight indica- 

 tions of rising, and if the stimulation is again repeated they 

 suddenly push up to the top of the spine. Thus the first effect of 

 a stimulus is to throw the rosettes into a certain preparatory 

 physiological state. While in this state a repetition of the stimu- 

 lus causes them to rise. 



In rare cases the rosettes are already in this irritated condi- 

 tion, owing to the general bodily condition of the animal, so that 

 they rise to the attack at the first touch. This will be spoken of 

 later. 



Usually therefore, objects which come against the starfish and 

 remain quiet are not attacked by the pedicellariae. If we press 

 steadily on the tip of one of the spines, the rosettes do not rise, 

 though if we draw a hard body across the tip of the spine, or if 

 we strike it several times, they rise to the attack. If we drop 

 sand on the surface of the starfish, the grains are not seized by 

 the pedicellariae, even though the sand may cover a number of 

 the delicate gills. Any fine flocculent material on the surface of 

 the animal is left undisturbed, — though we may cause the pedi- 

 cellariae to seize it by disturbing them with a needle. Any small 

 animal in contact with the starfish is safe so long as it remains 

 perfectly quiet. Of course, this rarely happens, and as soon as 

 the animal begins to stir about, it is seized. If a small object 

 which has been seized by one of the pedicellariae j^ulls, this 

 causes all the pedicellariae of the rosette to rise. Thus the pedi- 



